Multiple traits associated with salt tolerance in lucerne: revealing the underlying cellular mechanisms

Salinity tolerance is a complex trait inferring the orchestrated regulation of a large number of physiological and biochemical processes at various levels of plant structural organisation. It remains to be answered which mechanisms and processes are crucial for salt tolerance in lucerne (Medicago sativa L.). In this study, salinity effects on plant growth characteristics, pigment and nutrient composition, PSII photochemistry, leaf sap osmolality, changes in anatomical and electrophysiological characteristics of leaf mesophyll, and net ion fluxes in roots of several lucerne genotypes were analysed. Salinity levels ranged from 40 to ~200 mm NaCl, and were applied to either 2-month-old plants or to germinating seedlings for a period of between 4 and 12 weeks in a series of hydroponic, pot and field experiments. Overall, the results suggest that different lucerne genotypes employ at least two different mechanisms for salt tolerance. Sodium exclusion appeared to be the mechanism employed by at least one of the tolerant genotypes (Ameristand 801S). This cultivar had the lowest leaf thickness, as well as the lowest concentration of Na⁺ in the leaf tissue. The other tolerant genotype, L33, had much thicker leaves and almost twice the leaf Na⁺ concentration of Ameristand. Both cultivars showed much less depolarisation of leaf membrane potential than the sensitive cultivars and, thus, had better K⁺ retention ability in both root and leaf tissues. The implications of the above measurements for screening lucerne germplasm for salt tolerance are discussed.